It is pretty hard to get a perfect fit "each and every time".
Before I complete the turning, I measure both the blank and the nib (and finial) very close to where they will meet, using a digital caliper. . I measure several times (say 5) and take an average of the measurements because it is unlikely you will get the same value every time.
Then I turn the wood to 0.002" smaller in radius than the size of the nib where it will meet the wood.
Remember that your caliper measures the diameter not the radius so you will be constantly making adjustments for that fact.
I try to avoid sanding close to the ends of the wood where it will meet the metal.
By undersizing the wood at the ends by 0.002" (radius) this allows me to put on about 5 or 6 coats of thin CA (of the brand that I use) to raise the wood to about the nib size. But I usually put on about 8 coats of thin CA and then sand it back down to size. . Fewer coats for medium CA
In my opinion, it does not matter if the resulting wood + CA is a little oversize, but I do not like it to be undersize compared to the nib.
All comes down to your own materials, experience, and a bit of luck.
You will have to rely on trial and error ... and you will make mistakes, but eventually you will develop a technique that works for you most of the time.
If you want to sand your whole blank then you will have to take into account how much material the sanding takes off. . Make a trial pen and measure the important areas before and after sanding. . It should be an actual pen (not just a round rod) because when you are sanding, you will usually take off more at the ends than you do in the middle of the blank. . Keep written records of wood type, sanding grit, how many sanding strokes, what part of the blank the measurement was made at, etc.
When I am sanding, I always have the lathe turned off and I always sand in the lengthwise direction of the blank, turning the lathe headstock slowly by hand as I am sanding. . If you sand with the lathe on you don't have good control of how much material you are taking off, AND you will be sanding circumferentially which leaves scratches around the blank that are hard to get off later. . Sanding with the lathe on is OK for roughing to size, but the fine sanding should be done with the lathe off and always done lengthwise.
You can buy a decent digital caliper at a tool store for around $30. . I would not buy anything priced under $20. . Try it out before you buy it to check that it operates smoothly and do not buy one that has a capacity of less than 6.000". . (you will want to measure tube and blank lengths sooner or later) Make sure that the battery in it is a type that is readily available at more than one store because you will need to replace it at some time.